1. Field of the Invention
The invention relates to a multilayer glass ceramic substrate suitable for arranging LSI elements in high density, and more particularly to a multilayer glass ceramic substrate which can be fired at relatively low temperature and a method for fabricating the same.
2. Description of the Related Art
With the development of semiconductor technology, an electronic device is more required to be smaller in size and operate at higher speed. In the field of a semiconductor element, since VLSI and ULSI are highly densified and integrated, the technology for assembling these semiconductor elements is required to handle extremely densified and refined components. In particular, a substrate on which semiconductor elements are to be mounted is required to have low dielectric constant and be able to have highly densified leading wire arrangement thereon in response to the requirement of reducing leading wire diameter due to increased density of leading wire arrangement, and also in response to the requirement of reducing resistance of leading wires and the requirement of high operation speed.
An multilayer substrate made of aluminum oxide (aluminum oxide is generally referred to as "alumina") has conventionally been used. This multilayer substrate can be fabricated by thick film printing multilayer process or green sheet lamination process. The green sheet lamination process is detailed, for instance, in U.S. Pat. Nos. 3,423,517 and 3,723,176. The green sheet lamination process is more preferable for accomplishing highly densified arrangement. In the green sheet lamination process, leading wires are printed on each thin ceramic green sheet layer, and then layers are integrally laminated. Accordingly, it is possible to increase a number of layers on which leading wires have been printed, enabling highly densified arrangement of leading wires relative to thick film printing multilayer process. However, since alumina ceramic has the firing temperature above 1500 degrees centigrade, it is necessary for leading wires to use metals having relatively high electrical resistance such as Mo and W. This increases difficulty in reducing diameter of leading wires.
Recently ceramic material which can be fired at relatively low temperature has been developed in order to make it possible to use low-resistive conductors such as Au, Ag-Pd, Ag and Cu. A compound composed of alumina and borosilicate lead glass can be fired at the temperature below 1000 degrees centigrade, and hence a multilayer substrate in which Au, Ag-Pd or Ag is used as leading wires has been developed. Since this compound contains lead, it is impossible to fabricate leading wires of Cu which is a base metal, and in addition thereto, the dielectric constant of the compound can be reduced merely to 7.5 at least.
Another glass ceramic material including borosilicate glass has been developed for the purpose of decreasing dielectric constant and making it possible to fire or bake in reducing atmosphere at the temperature below 1000 degrees centigrade. This material can actually decrease the dielectric constant to about 5.5, and enables multilayering using Cu leading wires by virtue of a process of concurrently firing glass ceramic together with conductors. However, this material has a deficiency that physical strength thereof extremely lowers because of absence of crystallization during being fired.
For instance, Japanese Public Disclosure No. 54-111517 based on U.S. Patent Application No. 875,703 filed on Feb. 6, 1978 in the United States and assigned to International Business Machines Corporation, which was published in Japan on Aug. 31, 1979, discloses non-porous glass ceramic material. This material also has the aforementioned deficiency.
As aforementioned, since conventional alumina multilayer substrate can be fired only at relatively high temperature, high-resistive metals such as Mo and W have to be used as conductors. Accordingly, the resistance of leading wires cannot avoid from increasing, and in addition thereto, it is impossible to reduce the diameter of leading wires. Furthermore, in the conventional substrate, the dielectric constant thereof is relatively high, specifically about 10, and hence it is difficult to transmit signals at high speed.
The aforementioned compound composed of alumina and borosilicate lead glass can be fired at low temperature, and hence low-resistive conductors can be used as leading wires. However, it is quite difficult to fire the compound in reducing atmosphere and use conductive leading wires composed of base metal.
In addition, though the glass ceramic substrate including borosilicate lead glass enables an arrangement of multilayer leading wires made of Cu and decrease of dielectric constant, the physical or mechanical strength of the substrate is enormously lowered. The mechanical strength is quite important characteristic to the substrate. In particular, in a multi-chip substrate on which a lot of semiconductor elements are to be mounted, the substrate has to be enlarged in size and a lot of input and output terminals or pins are connected to each other or another elements. Therefore, there often arise problems of broken substrates and inappropriate connection with metals not only during assembly steps but also after the substrate has been completed.